As a high-efficiency amplifier, a Doherty amplifier has been known, for instance. The Doherty amplifier generally has two amplifiers, a carrier amplifier and a peaking amplifier, out of which the carrier amplifier is biased to operate in class A or class AB and the peaking amplifier is biased to operate in class C. That is, a bias of the peaking amplifier is set deeper than a bias of the carrier amplifier. Further, the Doherty amplifier includes, at a carrier amplifier output, a transmission line having a 90 degree (λ/4) electrical length in terms of carrier frequency, and by load modulation, it realizes a high-efficiency operation over a wide output power.
On the other hand, for a signal with a frequency different from the frequency corresponding to the transmission line connected to the carrier amplifier output, the operation of the amplifier falls out of a desired characteristic. That is, a wide band operation of the whole amplifier has been difficult.